1. Field of the Invention
The present invention relates to a magnetic bearing arrangement which is applied to a floating object, for example, a high-speed rotor such as a spindle for a turbomolecular pump, compressor, turbine or machine tool.
2. Description of the Prior Art
As a means for holding a high-speed rotor as a floating object in a floating condition there is known a magnetic bearing making use of electromagnets. Referring now to FIG. 5, such a magnetic bearing of the prior art will be described. Reference numeral 4 designates a rotor, numerals 5a and 5b designate a pair of magnetic bearings (journal bearings) for rotatably supporting the rotor 4, and numerals 6a and 6b each designates a position sensor for detecting the floating position of the rotor 4.
The amperage of the electric current to be passed through electromagnets (not shown) provided on the magnetic bearings 5a and 5b is determined on the basis of a detection signal obtained by the floating position of the rotor 4 being detected by the position sensors 6a and 6b, thereby controlling the magnitude of the magnetic force generated in the electromagnets so that the floating position of the rotor 4 is set.
FIG. 6 is a block diagram of a control apparatus for the aforementioned magnetic bearings. In FIG. 6, reference numeral 1 designates a magnetic bearing control apparatus, numeral 2 designates a structure (such as a casing) having a rotor, numeral 3 designates a portion in which interchangeability is restricted, numeral 7 designates a position sensor, numeral 8 designates a central position (off-set) regulator, numeral 9 designates a positional feedback gain adjuster, numeral 10 designates a control circuit tuned to the natural frequency of the rotor 4, which is hereinafter called "a filter", numeral 11 designates another control circuit, and numeral 12 designates an electromagnet.
The central position (off-set) regulator 8 serves to electrically correct and adjust mechanical dislocation in the floating and holding center of the rotor, and depends upon error in assembly that occurred at the time of placing the position sensor 7. It is to be understood that the adjustment value thereof will vary for each piece of equipment having magnetic bearings.
The positional feedback gain adjuster 9 serves to convert the output voltage of the position sensor 7 into a displacement of the rotor 4 from its floating center, and it further includes an operation of correcting
the sensitivity of the position sensor 7 that differs at its respective points.
The filter 10 serves to damp a high frequency hunting (natural vibration of rotor bending mode) which is generated because of a lower damping capacity based on a high gain and phase lag of a magnetic bearing in its high frequency region, thereby ensuring the rotative stability thereof. It is also to be understood that the set value of the central frequency of this filter 10 will vary for each piece of equipments having magnetic bearings in order to fit with the natural frequency of the rotor 4.
The control circuit 11 is generally composed of a proportioning element, an integrating clement and a differentiating element (PID clement) in order that a positional control for a deviation of the rotor 4 from its floating center and a gain and phase designed to damp the oscillation of the rotor 4 having a natural value of rigid body mode can be ensured.
The central position (off-set) regulator 8, positional feedback gain adjuster 9, filter 10 and control circuit 11 are usually incorporated in the magnetic bearing control apparatus 1, while the position sensor 7 and electromagnet 12 are incorporated in the structure 2 having the rotor.
In the aforementioned prior art, there are the following problems to be solved.
(1) In general, equipment having magnetic bearings comprises two units of a structure having a rotor and a magnetic bearing control apparatus for controlling the rotor as it is floated. Of course this magnetic bearing control apparatus includes an inverter for high-speed rotation of the rotor.
The aforementioned two units are connected with each other by means of a magnetic bearing controlling cable and a motor driving cable.
(2) The magnetic bearing control apparatus includes portions which are regulated or set depending on the properties of the structure having the rotor. For example, the central position (off-set) regulator 8 for correcting a mechanical dislocation in the floating center of the rotor, depending upon error in assembly occurring at the time of placing the position sensor 7, the positional feedback gain adjuster 9 for converting the output voltage of the position sensor 7 into a deviation of the rotor from its floating center, and the filter 10 for damping the oscillation of the rotor having a primary natural value of bending mode, correspond to these portions.
Adjustment values and set values in these portions which are regulated or set depending on the properties of the structure will vary on each of the structures having the rotor. Therefore, the magnetic bearing control apparatus and the structure having the rotor make a one-to-one combination.
(3) If this one-to-one combination which the magnetic bearing control apparatus and the structure having the rotor make is changed, normal function would be lost in the control of the magnetic bearing, and as a result its stable rotation could not be ensured.
(4) The fact that this one-to-one combination can not be modified, i.e. the fact that there is no interchangeability between the magnetic bearing control apparatus and the structure having the rotor, reduces remarkably the mass-productivity of equipment having magnetic bearings. Furthermore, this fact is undesirable for managing such equipment in after-sale service or for taking countermeasures to some problems.